This invention relates generally to improved well logging and perforating apparatus, and, more specifically relates to apparatus for decentralizing and orienting a well logging or perforating instrument relative to one side of a borehole.
In virtually any earth borehole, even those intended to be perfectly vertical and drilled accordingly, there will be at least a slight deviation from true vertical, yielding a high and low side to the borehole across a plane perpendicular to the borehole at any given depth. In deviated or inclined boreholes, this deviation from vertical is, by definition, more pronounced, thereby more clearly allowing the borehole to evidence high and low, or top and bottom, sides. In many well operations, particularly perforating operations, it is often desirable to locate the instrument against the borehole casing and to orient the instrument relative to the casing such that a certain side of the instrument will run along this bottom side of the borehole. This orientation relative to and against the casing is particularly useful in perforating operations.
In perforating operations a gun containing explosive charges is lowered into the borehole to a predetermined depth where the charges are fired, causing them to penetrate the borehole casing and the surrounding earth formations. One advantage of orienting the perforating gun relative to the borehole is that when these charges are fired with the perforating gun positioned such that the side from which the charges will emerge is located against the casing, the charges will penetrate the casing with a clean entrance hole rather than leaving a burred rim on the interior of the casing around the hole. Further the locating of the perforating gun against the borehole sidewall reduces the radial movement of the gun within the borehole in reaction to the explosive detonations, thereby reducing the likelihood of damage to the perforating gun. This post-detonation movement is further reduced when a means is provided for maintaining the positioning of the gun against the borehole sidewall.
The oil and gas industry has predominantly utilized devices of two classes for attempting to achieve the above-described orientation and decentralization. The first class is composed generally of devices consisting of a bow-spring affixed to and extending from one side of the logging or perforating instrument such that the spring exerts a force on the borehole sidewall, forcing the instrument against the opposite sidewall. The weight of the instrument is relied upon to orient the instrument to the bottom side of the borehole. Experience has shown these bow-spring devices are unweildy to handle, and may present difficulties when traversing wellheads or lubricators affixed to the top of the well. Additionally, these bow-spring orienting decentralizers show a tendency to bind in the borehole when attempting to traverse bends or reductions of diameter in the borehole.
The second class of orienting decentralizers consists generally of devices of an essentially cylindrical form with magnets mounted along one side. These devices orient solely by means of the magnets exerting an attractive force upon the well casing, therefore they have virtually no orienting capability in an uncased borehole. Further, with this class of orienting decentralizers, if the orienting decentralizer and attached instrument lie in an orientation such that the casing is not sufficiently acted upon by the magnets, the instrument, although resting on the lower side of the casing, will not be directed to the desired rotational orientation relative to that lower side of the casing. Further, the more closely the radius of the casing approaches the radius of the decentralizer, the lower the potential for the magnets, if they do contact the casing, to orient to the direct bottom or lower side of the casing and to similarly orient the attached instrument.
Accordingly, the present invention overcomes the deficiencies of the prior art by providing an apparatus which reliably orients a logging or perforating instrument relative to the bottom of a borehole and maintains such instrument in a predetermined rotational relation to such borehole.